This invention relates generally to computer-controlled winch systems for wireline logging. More particularly, the invention is a computer-controlled heave compensation wireline logging winch system that compensates for the effects of wave motion on floating installations performing wireline logging.
Wireline logging is the process by which oil or gas wells are surveyed to determine their geological, pertrophysical or geophysical properties using electronic measuring instruments conveyed into the wellbore by means of an armored steel cable, known as a wireline cable. The wireline cable is stored on a winch drum, which provides the mechanism by which it is lowered into the well via a series of sheave wheels to ensure proper alignment. The measurements made by downhole instruments secured to the wireline cable are transmitted back to a data acquisition computer located at the surface through electrical conductors in the wireline cable. Electrical, acoustical, nuclear and imaging tools are used to stimulate the formations and fluids within the wellbore and the electronic measuring instruments then measure the response of the formations and fluids. A device mounted close to the cable drum at the surface determines the depth at which these measurements are recorded. This device measures cable movement into and out of the well and is known as the depth system. The wireline well log contains the record of the series of measurements of the formations and fluids found in the wellbore with respect to the location within the borehole at which measurements are made. The raw measurements are often presented in the form of an x-y graph with the location where the measurement is made recorded on the y-axis and the measurement itself recorded on the x-axis. The location where the measurement is made is called the depth. It is a measure of the distance between a reference position, usually located somewhere on the surface above the well, and the location within the borehole following the path of the borehole.
The accuracy and quality of the wireline logging data obtained from such an arrangement is dependent on the smooth movement of the wireline cable and the downhole logging tools that extend from the wireline cable at a known and controlled speed, along with the precise determination of the depth at which the wireline logging measurements are made. Depth may be calculated by measuring the amount of cable spooled off or on the winch and may be adjusted for conditions in the borehole and characteristics of the cable. One cable characteristic that may be adjusted for is cable stretch, which is a function of temperature, pressure, tension and length of the cable.
For a fixed wireline setup, such as a land drilling rig or fixed offshore platform, the measurement of depth and cable speed is relatively straightforward. This is because the variables in the system can be measured and accounted for. On a land rig or fixed drilling rig, there is a fixed distance between a reference point at the surface of the well itself and the winch. Because the distance is fixed, it may be automatically adjusted out of the depth calculation. However, when the winch is installed on a floating vessel, which may typically be a semisubmersible rig, drill ship or barge, the movement of the rig itself due to tidal or wave motion effects is not taken into account by conventional wireline logging systems. In a floating vessel installation, the distance between the reference point at the surface of the well and the winch is not fixed and the distance changes with respect to the tide and waves. If ignored, the vertical component of this motion, relative to the wellbore, will have an adverse affect on the indexing and analysis of the log data. The movement of the wireline cable and the downhole logging tools induced by the movement of the rig, drillship or barge will not be measured. This same problem occurs if the rig is fixed, but the wireline winch is located on a floating tender.
Other systems have attempted to minimize the effects of wave motion on wireline logging data. The system is often compensated in such a way as to keep the wireline set-up fixed with respect to a known reference datum, usually the sea floor. This is normally achieved by interfacing with the drilling rig's compensation system, and using it to anchor the wireline rig to the fixed datum. A compensation device, usually in the crown of the rig, attempts to hold the cable distance constant using an electro-hydraulic device. This system is limited in its precision and the range of motion over which it can compensate since it relies on a passive compensation system designed for very heavy drill pipe strings and uses steel ropes to anchor the wireline upper sheave wheel to the seabed. The wireline acquisition system then assumes that the setup is not changing and is fixed. This type of system is high maintenance and expensive. Alternatively, an electromechanical compensation device can be inserted between the winch and the upper sheave wheel to be used for well logging only. Since well logging is done somewhat infrequently, this device is often idle. In both of these types of systems, no corrections are made for any errors induced by incomplete heave compensation.